The present invention relates to a method of compensating for drift within a fire detector, to a fire detector arranged to compensate for drift, and to a fire detector system.
After installation, it is known that most fire detectors will deteriorate over a period of time, and their response characteristics will change, normally owing to a build-up of dirt. This is particularly acute when a detector is located outside or in a location exposed to a large amount of dust or dirt. In such locations, it might be necessary to replace or clean the fire detector on a very regular basis to ensure that its response remains within acceptable limits. Such a fire detector will detect the characteristics of a fire, such as the presence of smoke particles, radiation of a certain wavelength indicative of a fire or heat, and when the characteristics exceed a threshold, an alarm will be signalled.
For example, a known type of smoke detector includes a detector chamber defining a detection region which is shielded from the light of the surrounding area, but which permits entry of air into the detector chamber. The detector chamber includes a light source, typically in the form of an LED, and a light detector, each of which is separated from the other by structural features which mean that light cannot pass directly from the light source to the light detector. In the event that a fire occurs, smoke will be carried into the detector chamber by the air, and light from the light source will be reflected or scattered from the smoke particles. The reflected or scattered light will be detected by the light detector, the amount of reflected light being indicative of the amount of smoke in the air within the detector chamber. Thus, as the amount of smoke in the air increases, the response of the fire detector increases accordingly.
Over time, dirt gradually builds up in the detector unit, for example on the internal walls of the detector chamber. Therefore, over a long period of time, the light detector is able to detect light which reflects off the dirt which has built up on the detector walls, giving a positive response. This can be seen in FIG. 1 in which the detector output signal is plotted against time. Over a long period of time, the response from the detector output signal gradually increases.
It will be appreciated that, if there is a fire in the vicinity of the smoke detector, the output signal of the detector will significantly increase as smoke enters the detector chamber until it crosses a pre-set value which is the alarm threshold. When it crosses the alarm threshold, an alarm is signalled, and a siren will sound. It will be appreciated from FIG. 1 that, when the smoke detector is first commissioned, the amount of smoke that must enter the detector chamber before the alarm threshold is crossed will be sufficiently large that the alarm is not triggered by other environmental conditions which might cause a false alarm. This is indicated by double-headed arrow A. Later in the detector's life, the sensitivity of the detector actually increases because the amount of smoke or airborne particles required to trigger an alarm condition is much less, as indicated by double headed arrow B. The point comes where the sensitivity is increased to a level where the likelihood of a false alarm being triggered by a non-fire condition becomes too great.
Of course, in the smoke detector described above, over time, the detector becomes more sensitive. Other types of detector become less sensitive with age and with the build up of dirt. In the case of a detector which becomes less sensitive, the response might be indicated in a graph with a response which gradually drops over time, rather than increases, so that the sensitivity decreases rather than increases.
It is known to compensate for the change in response of a fire detector over time by gradually changing the alarm threshold which must be crossed for an alarm to be triggered. The rate at which the threshold is changed can be set depending on the environment in which the fire detector is installed. In a very dirty environment, the threshold will be changed relatively quickly compared with a fire detector installed in a relatively cleaner environment. The change in the threshold is either implemented in the detector hardware or, more commonly, within a software algorithm running within the detector. The response of a smoke detector similar to the one described above is shown in FIG. 2, in which the alarm threshold increases in line with the response from the detector. As a result, the sensitivity remains the same over a long period of time as is indicated by double headed arrow C which indicates a distance between the detector output signal and the alarm threshold.
Of course, the point is reached when the fire detector is no longer able to resolve a fire alarm because the end of the dynamic range of the detector has been reached. Any further change in the threshold would mean that the fire detector would not be able to resolve a fire alarm, thereby risking the missing of a real alarm event, and this point is known as the drift compensation limit. The fire detector may be arranged so that, prior to reaching the drift compensation limit, a pre-warning is given to allow the fire detector to be replaced or cleaned. When it actually reaches the drift compensation limit, the fire detector is arranged to signal a warning or a fault.
It will be understood, therefore, that after a period of time, the drift compensation limit will be reached, and the fire detector must be cleaned or replaced. In a dirty environment, this point might be reached quite quickly, and this represents a regular expense. It would be advantageous if the frequency of cleaning or replacing the fire detector can be reduced.